Unconventional Magnetism and Band Gap Formation in LiFePO4: Consequence of Polyanion Induced Non-planarity

Abstract

Oxygen plays a critical role in strongly correlated transition metal oxides as crystal field effect is one of the key factors that determine the degree of localization of the valence d/f states. Based on the localization, a set of conventional mechanisms such as Mott-Hubbard, Charge-transfer and Slater were formulated to explain the antiferromagnetic and insulating (AFI) phenomena in many of these correlated systems. From the case study on LiFePO4, through density-functional calculations, we demonstrate that none of these mechanisms are strictly applicable to explain the AFI behavior when the transition metal oxides have polyanions such as (PO4)3−. The symmetry-lowering of the metal-oxygen complex, to stabilize the polyanion, creates an asymmetric crystal field for d/f states. In LiFePO4 this field creates completely non-degenerate Fe-d states which, with negligible p-d and d-d covalent interactions, become atomically localized to ensure a gap at the Fermi level. Due to large exchange splitting, high spin state is favored and an antiferromagnetic configuration is stabilized. For the prototype LiFePO4, independent electron approximation is good enough to obtain the AFI ground state. Inclusion of additional correlation measures like Hubbard U simply amplifies the gap and therefore LiFePO4 can be preferably called as weakly coupled Mott insulator.

Energetics and cathode voltages of LiMPO4 olivines ( M=Fe , Mn) from extended Hubbard functionals

“…This ordering is consistent with previous work in literature [157,160] (at least for the ground and first-excited states) and is robust with respect the particular approximation being used, although the energy differences between the various magnetic orders varies with the specific flavor of Hubbard correction in use. For example, for the PBEsol functional the energies of AF 2 and AF 3 are about 10.8, 19.3 meV/f.u.…”

“…This cell is large enough to accommodate a few antiferromagnetic states and Li configurations which will be compared in the last part of this section. While the energy scale of inter-atomic magnetic interactions makes the comparison of various AFM configurations marginally relevant for the energetics of the material (in particular, for its average voltage and the formation energy of the half-lithiated compound) especially at finite temperatures, this comparative analysis is still interesting to better understand the electronic structure of cathode materials [157] and, in general, to improve their characterization (in comparison with experiments) [158,159], especially if a significant shift of magnetic properties (e.g., the Néel temperature) is expected to correlate with Li intercalation. All the calculations shown in the first part are performed for the antiferromagnetic ordering (named AF1 in the later discussion) that was verified to correspond to the ground state configuration.…”

DFT investigation of the effect of spin-orbit coupling on the NMR shifts in paramagnetic solids

“…While comparison with experimental ∆g iso (Table III) might suggest improved agreement for DFT+U, this would indeed be due to compensation with other errors. We thus have to conclude that, while DFT+U improves hyperfine interactions, magnetic moments, band gap, and other aspects of electronic structure in the present LiTMPO 4 systems [63,[82][83][84][85][86], it is not suitable for response properties, unless one finds a way to mimic the response coupling terms. A correct treatment of solid-state g-tensors with hybrid functionals may provide a better route for future improvement.…”

Engineering Diffusivity and Operating Voltage in Lithium Iron Phosphate through Transition-Metal Doping

“…M x PO4 . This is due to the fact that even though the Olivine phosphates are crystalline solids, the Bloch d electrons behave like atomically localized electrons[14]. To substantiate it further we have plotted the atomic M-d energy levels inFig.…”